Make async lookups ignore remote apps

Fixes http://manage.dimagi.com/default.asp?142325
so the memoization should help a lot but this is now making one request to couch per app instead of one for every 100 (or whatever `iter_docs` default is). it's a bit coupled to the implementation, but the performance win might be better if you used `iter_docs` still and then filtered out just using `get_correct_app_class` and then the same `is_remote_app` check.